The present invention relates to a liquid droplet generator and, more particularly, to a high energy, acoustic droplet generator capable of creating high amplitude velocity perturbations on a stream of fluid which are sufficient to atomize the fluid into a stream of droplets.
The atomization of a jet or sheet of liquid is a process which, in most cases, requires energy to be added to the liquid. The added energy is converted into an increase in surface energy in the liquid as the initial liquid mass is separated into droplets. As the surface energy of the liquid increases, the surface area of the liquid likewise increases. Energy may be supplied for purposes of atomization from either a decrease in kinetic energy of the liquid or from an external source.
One prior art process for atomizing a fluid involves impinging a fast moving air stream onto a slower moving fluid, such as a fuel to be burned in a combustor of a turbine engine. With this process, the kinetic energy of the injected air serves to tear the liquid into filaments and then into drops. Thus, a portion of the kinetic energy of the injected air is converted into an increase in surface energy in the atomized fluid.
The prior art air injection process, when used to atomize a fuel to be burned in a turbine engine, is only effective when the engine is operating, since a source of high velocity air is needed for atomization. Further, higher engine operating temperatures, which result in greater engine operating efficiency, are difficult to achieve since excess air is added into the engine for purposes of atomization. Additionally, atomization by use of injected air results in an inconsistent distribution of fuel spray in both time and space. As a result, the combustor is required to be longer than otherwise necessary to ensure that all the fuel is burned before the air/fuel mixture exits the combustor. The inconsistent distribution of fuel spray also results in a non-uniform combustion of the air/fuel mixture causing an increase in NOx pollutants being emitted from the engine.
A further prior art atomization process involves the acoustic excitation of a circular liquid jet at an unstable wavelength. Rayleigh explained in 1878 that a circular fluid jet is unstable for azimuthally symmetric perturbations whose axial wavelength is longer than the circumference of the jet. This prior art process is based upon Rayleigh's theoretical work. The process involves placing small amplitude acoustic perturbations on a circular jet, wherein the perturbations have a wavelength longer than the circumference of the jet. The applied perturbations grow, due to an input of energy from surface tension, and break the jet into a stream of drops at the excitation frequency. This process adds little or no energy to the fluid. Thus, the surface area and surface energy of the fluid is lower after break-up than before. Further, the size of the resulting drops produced by this process have a diameter approximately twice the diameter of the original jet. Thus, if small drops are desired, small nozzles or orifices must be used. Small nozzles, however, can be easily obstructed by particles carried by a fluid. Consequently, this process is disadvantageous for use where small droplets are desired. Further, this process will not induce atomization of a sheet of liquid.
Accordingly, there is a need for an apparatus which is capable of adding energy to a liquid stream for purposes of atomization without employing high velocity air. There is a further need for an apparatus capable of employing acoustic energy for atomizing a liquid stream into a stream of droplets having a greater surface area and surface energy than that of the initial stream, and which is further capable of inducing atomization of a sheet of liquid.